ASK THE DOC — MGUS and spot on PET scan turns out negative

Patient has had MGUS and is sta­ble. A chest x‑ray showed a spot near the heart on the left low­er lung. Patient was lat­er sick and in hos­pi­tal for a week and the same area was spot­ted on PET scan. When doing addi­tion­al tests the lump was not seen. The ques­tion is what could it be and what should they do.